The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.

THE DOLLAR BAY FORMATION OF
LOWER CRETACEOUS (FREDERICKSBURG) AGE IN
SOUTH FLORIDA

ITS STRATIGRAPHY AND PETROLEUM POSSIBILITIES

By
George O. Winston

Published for
BUREAU OF GEOLOGY
DIVISION OF INTERIOR RESOURCES
FLORIDA DEPARTMENT OF NATURAL RESOURCES

TALLAHASSEE
1971

Completed manuscript received
February 20, 1970
Published by the Bureau of Geology
Division of Interior Resources
Department of Natural Resources

ii

September 27, 1971

The Honorable Reubin O'D. Askew
Governor of The State of Florida
The Capitol
Tallahassee, Florida 32304

Dear Governor Askew:

The Bureau of Geology, in accordance with Florida Statute No. 373, Paragraph
3, is publishing as Special Publication No. 15, "The Dollar Bay Formation Of
Lower Cretaceous (Fredericksburg) Age In South Florida Its Stratigraphy And
Petroleum Possibilities" as prepared by Mr. George O. Winston.

This evaluation of this particular group of sediments provides a quick and
valuable review of a difficult geological section as to its potential for the
production of hydrocarbons. We are pleased indeed to have this report made
available to us at this particular time when the exploration for oil is so
concentrated in Florida.

The writer is indebted to the Bureau of Geology, Division of Interior
Resources, Florida Department of Natural Resources, for making available
samples and electric logs used in this study.
Members of the Bureau of Geology to whom the writer wishes to express his
appreciation are: Dr. R.O. Vernon, Chief, for his encouragement and aid; C. V.
Babcock for assembling data and materials and for editing the text of the report;
and W. R. Oglesby, whose unpublished cross-sections and correlations expedited
the investigation.
Appreciation is extended to J. E. Banks of the Coastal Petroleum Company,
Tallahassee, Florida, who made available to the writer samples and data on
various wells.
Appreciation is here expressed to E. K. King, P. L. Lyon, and D. I. Gough for
permission to reproduce portions of their published geophysical maps.

THE DOLLAR BAY FORMATION
OF LOWER CRETACEOUS (FREDERICKSBURG) AGE
IN SOUTH FLORIDA

ITS STRATIGRAPHY AND PETROLEUM POSSIBILITIES

By
George O. Winston

INTRODUCTION
STUDY AREA

The area of the report consists of the onshore portion of the South Florida
Basin, figure 1. Approximately 70 oil tests have been drilled there, 17 of which
provided reliable cores and samples which were utilized in the preparation of this
report. Geographical localities in south Florida are shown on figure 2.

PURPOSE

The purpose of this study is to describe, subdivide, and map the rocks of the
Dollar Bay Formation of Fredericksburg Age in south Florida and to outline
areas favorable for oil accumulation.

METHOD

Drill cuttings and core chunks were examined by the writer using a 10-power
stereoscopic microscope. Wells which had extensive coring or very good quality
samples were used where possible. All rock samples were examined wet and by
incandescent light.
Electric log tops were picked to the nearest 5 feet; all correlations were made
by the writer. Isopachous maps were constructed using electric log intervals;
facies data are from sample descriptions made by the writer. Grain and crystal
sizes were determined according to the Wentworth scale as modified for
carbonates by Leighton and Pendexter (1962, P. 52).
The detailed discussions of textural types, porosity types, cyclic
reconstruction and other features requiring accurate detail are based on the wells
which had plentiful cores. Drill cuttings were used to trace lateral lithologic
changes where core information was not available. Knowing the cyclic sequence
from core study, 10-foot sample intervals were adequate even to describe thin
cycles. Wells with 20-foot samples were used only where a control point was

badly needed. Wells with 30-foot samples were never used, as detailed lithologic
determination is impossible with such a gross interval. Unfortunately, most of
the wells in south Florida fall in the last group.
Appendix 5 tabulates all wells used on the maps. Appendix 6 is a table of
lithologic control wells used in this study. Appendix 7 shows wells which were
found to be useless for lithologic control in the Dollar Bay interval.

SCOPE

This report comprises one-third of a completed study of rocks of
Fredericksburg Age in south Florida. The other two-thirds of the study will be
published at a later date.
This portion of the study describes and maps the texture and facies
distribution of the carbonates in the Dollar Bay Formation. The percent of
anhydrite in the section was mapped because it is a common constituent of the
formation, but a textural study of this lithology was not undertaken; such a
study would be a project in itself. Clastic sediments were not mapped because
they are very rare.
Copies of the sample descriptions and data tabulations are on file at the
Bureau of Geology, Division of Interior Resources, Florida Department of
Natural Resources, Box 631, Tallahassee, Florida 32302.

STRUCTURE

The area covered in this report is part of the extensive Florida-Bahama
Platform, most of which is submarine. This feature encompasses the area
between the Florida escarpment to the west, the Blake-Bahama escarpment to
the east, and the north coast of Cuba to the south. During Cretaceous time the
platform was without known major structural anomolies, such as deep oceanic
straits or channels.
The South Florida Basin has been mapped by Oglesby (1965) on top of the
Sunniland Limestone. This slowly subsiding low-dip basin of carbonate and
anhydrite accumulation which is centered in the vicinity of Florida Bay,
continued as a prominent feature of the platform during deposition of the Dollar
Bay Formation; however, the onshore configuration of the feature was
somewhat altered with time. Figure 3 outlines the onshore portion of the South
Florida Basin at the level of Dollar Bay sediments; the dip of the basin, which is
about 50 feet per mile, is so low that its boundaries cannot be defined precisely.
Nevertheless, the central part of the feature is shown on all of the facies, isopach
and structure maps of the report. It is suggested on geophysical maps, most of
which are based on limited data in this area.

SPECIAL PUBLICATION NO. 15

--*---1

I ( / i
EXPLANATION
Oil recovery on DST
Oil cut mudonOST
0 Good show in rock
o Poor show in rock
e No show reported -
) Lithologic Control Wells-
04o -

o0 042
'o, V

10 0 10 0 30 MILES
APMPOX.$CALE

Contour Interval 200 Feet

Figure 3. Structure map on base of Dollar Bay Formation.

BUREAU OF GEOLOGY

For the purpose of this study, the northern boundary of the South Florida
Basin is considered to occur along an east trending line across the north side of
Lake Okeechobee. To the east and south the basin terminates at the 600-foot
bathymetric contour at the edge of the Florida Straits. The western boundary is
not determinable from this study, but it may occur at the weakly developed
Lee-Collier swell which extends southwestward from Lee County, figure 4.
Alternatively, as shown by Oglesby (1965) for several horizons in the vicinity of
the Sunniland Limestone, it may extend as far west as the Florida escarpment; in
at least part of the escarpment seismic profiles and a dredged sample indicate
that a possible Cretaceous reef may occur at a present depth of about 4,800 feet
(Antoine et al, 1967, fig. 3, p. 259).

INFORMAL DESCRIPTION OF MAJOR STRUCTURAL FEATURES

Within the South Florida Basin, or bounding it, are several broad local
structural features (fig. 4) which were present during the deposition of the
Dollar Bay Formation. Some of the stronger features may be of basement origin
as they also appear on published regional magnetic and gravity maps (see
Appendix 8).
The Martin high is a strongly-developed southward-plunging feature which
terminates at the Broward trough and is present on most facies maps. This high
is either the south end of the Peninsular arch, which is the dominant feature of
central Florida, or an offshoot from it. The Broward trough (which is not to be
confused with the Broward syncline of the Applins) is a strong feature and
appears to originate in the Northwest Providence Channel in the Bahamas
(Winston, 1970). It also is very persistent on facies maps. This trough is shown
joining the central part of the South Florida Basin to the west on most maps,
although on some maps the Forty Mile Bend high intervenes between these two
negative features. The Forty Mile Bend high is a weak feature on most facies
maps, but shows up well on isopachous maps. The Pine Key arch, a westerly
offshoot of the Largo high, is a weak feature which is most prominent on
isopachous and structure maps; it is usually subdued on facies maps. The south
boundary of this feature is unknown. The Lee-Collier swell is a weak feature
near the western limit of well control. Though it is only vaguely indicated by the
data points, the concept of this feature frequently presents the most logical
solution to contouring problems. The regional magnetic map provides additional
support for the inferred presence of the Lee-Collier swell. As it is possible that
this feature forms the western boundary of the South Florida Basin, it is
unfortunate that its existence is subject to so much doubt.
The Charlotte high shows up well on the structure map (figs. 3 and 4), on
some isopachous maps, and on the regional magnetic map. The abnormally high
structural position of igneous rock in the Mobil No. 1 State 224B, offshore

SPECIAL PUBLICATION NO. 15 7

M A A TEE H ARD E E
\_ OKE HOBEE! ST.
j----- -- HIGHLANDS

DE SOTo .4 r
--\

0.

10 0 10 20 30 MILES
APPROX SCALE

Figure 4. Structural trends in south Florida.

BUREAU OF GEOLOGY

Charlotte County (well 14), indicates that this high reflects a basement feature.
Samples on this well were not available for examination.
There are no known major faults in the study area, but offshore faults have
been inferred from geophysical and bathymetric data in the Florida Straits to
the south.
Only two localities have had sufficient drilling to define a local subsurface
structure. At the Sunoco-Felda field in Hendry County (fig. 2) the structure of
the Sunniland Limestone, which is the producing horizon, is a southward
plunging nose; the perforated interval in productive wells varies from about 10
to 14 feet in thickness. As shown by Puri and Banks (1959, fig. 18, p. 130) for
the Sunniland field of Collier County, the proven anticlinal closure of the
productive Sunniland Limestone was 36 feet, and at the approximate level of the
Dollar Bay Formation the closure was 8 feet.

GEOLOGICAL HISTORY

The study area is part of the Florida-Bahama Platform, which was slowly
subsiding during Fredericksburg time. The center of greatest subsidence was
under the present Florida Bay, (Winston, 1970).
This negative area should contain most of the source beds for petroleum. The
occurrence of offshore Lower Cretaceous bioherms is suggested by published
marine seismic sections (Antoine et al, 1967, fig. 3, p. 259) along the Florida
escarpment to the west. Lower Cretaceous algal limestone has been dredged
from this same escarpment (Antoine et al, 1967, fig. 3, p. 259) and a photograph
along the escarpment strongly suggests a reef. The entire Florida-Bahama
platform was probably an immense back-reef shallow-water area throughout
most of the Lower Cretaceous. Supporting this statement are the omnipresent
Miliolidae, a~thydrite beds and the multiplicity of sedimentary cycles.

DESCRIIVE STRATIGRAPHY
PREVIOUS&WORK IN FLORIDA

Relatively little literature,.ha een directed toward the petroleum prospects
of Fredericksburg Age rocks in Florida. Previous publications have concentrated
on the oil-producing Sunniland Limestone of Trinity Age.
The Big Cypress Group, as formally described below, has long been known in
Florida by the term "Fredericksburg." This usage was pioneered by the Applins,
(1965, p. 56-60) who selected persistent electric log kicks near diagnostic fossil
zones to define the top and base of their unit. Their points were at the base of
two persistent anhydrite beds. In the present study, the tops of these anhydrite
beds (rather than the bases) are used, therefore the two sections are essentially
synonymous.

SPECIAL PUBLICATION NO. 15

The 1959 paper by Puri and Banks (p. 128, fig. 16) on the Sunniland field
shows a structure map on a Fredericksburg marker some 1900 feet above the
productive Sunniland interval.
Fredericksburg beds extend well beyond the limits of the study area.
Northward they pinch out against Paleozoic and igneous rocks on the crest of
the Peninsular arch (P. Applin, 1965, fig. 35, p. 57); they occur on the flanks
of this feature and finally pinch out in the subsurface of south Georgia and
Alabama. Only in northern Cuba do Lower Cretaceous beds appear at the
surface (Furrazola-Burmudez, 1964, p. 54).

REGIONAL CORRELATION

The rock name Big Cypress Group is herein proposed for the interval referred
to by authors and the petroleum industry by "Fredericksburg." The name was
taken from the Big Cypress Swamp near the type well. The interval of these
rocks extends from 9,850 to 11,208 feet, core depths (9,851 to 11,213 feet
electric log depths) in the Humble No. 1 Collier Corporation well, Sec.
27-T50S-R26E, Collier County, Florida. The Big Cypress Group includes the
Dollar Bay Formation, as defined below, as ~ell as an underlying sequence of
unnamed rocks. The entire group is composed of a cyclic series of limestone,
dolomite and anhydrite.
The relationship of the Big Cypress Group to stratigraphically equivalent units
occurring in other parts of the Gulf coast is presented in figure 5, a correlation
chart. In Louisiana, Mississippi and Alabama an undifferentiated plastic section is
equivalent to the Big Cypress Group of south Florida. In Texas, the Edwards,
Comanche Peak, Walnut, Goodland and Kiamichi formations are together
equivalent to the Big Cypress Group. According to a correlation chart by
Forgotson (1963, p. 72), the Paluxy Formation of Arkansas, Louisiana,
Mississippi and Alabama is of oldest Fredericksburg Age; thus it would be the
stratigraphic equivalent of the lowest part of the Big Cypress Group. Other
authors (Roy and Glockzin, 1941; Stephenson, et al, 1942; and Frascogna,
1957), however, consider the Paluxy to be of Trinity Age.
With the exception of ubiquitous Miliolidae, fossils in the Big Cypress Group
are scarce. The few diagnostic fossils which the Applins found and identified
indicate a Fredericksburg Age for these rocks.
These diagnostic fossils are:
Lituola subgoodlandensis (Vanderpool)
Coskinolinoides texanus Keijzer
Dicyclina schlumbergeri
Cyclammina
Calcisphaerula innominata Bonet
Chondrodonta munsoni (Hill)

BUREAU OF GEOLOGY

Z ULJZ Z2
cL ;=- 0 -

< < <

i co c i 2
7F

I| 1- I I-

< >-
I m- *I

S< E -

S LLJ mLu Z

o____ Z I \ IO_
OU z
< < O <

1 O 0
LLI-

VI A s s Sa y

NVaI1V
S3ddn

NVISlV 31ac11iA

NVISIV
aaMc1

SPECIAL PUBLICATION NO. 15

DISCUSSION OF LOWER CRETACEOUS ROCK TYPES

A review of the lithologic description of the Dollar Bay Formation in the type
well reveals that the formation consists of limestone, dolomite and anhydrite
deposited in a cyclic series. This cyclic sequence is typical of a large segment of
the Lower Cretaceous Section.
GENERAL LITHOLOGIC TYPES
LIMESTONE
There are two types of limestone present.
Calcilutite (appendix 1) is the more common of the two types. It is finely
microcrystalline in texture and is cream, tan brown, dark gray or dark brown in
color. Occasionally it displays a conchoidal fracture, a rubbly fracture, or a
lithographic texture. Chalky microcrystalline porosity is common, but it is
non-effective as an oil reservoir. Some of the calcilutite was formed from
calcarenite, as shown by instances of gradational destruction of texture by
recrystallization. Various types of carbonate grains occur in varying percentages
within the calcilutite.
Calcarenite comprises between 5 and 15 percent of the carbonate section in
south Florida. Voids in the calcarenite are filled to varying degrees with calcite
cement or with calcilutite. The types of calcarenite grains observed are:
1. Skeletal grains which range in size from micro-grain, well-rounded fossil
debris to very large irregular shell fragments or whole microfossils showing little
abrasion. The occasional occurrence of concentrations of rudistid fragments
indicates the possible presence of a nearby large local accumulation which might
form a porous shell bank similar to the producing horizons of the Sunniland
field.
2. Coated grains (including oolites) are occasionally present and vary in
size from very fine to medium grain. Generally there is only one coat over a
nucleus which is usually a skeletal grain or a miliolid foraminifera. Dolomite
crystals are sometimes found in the centers of oolite grains; this is most likely a
secondary growth, as advanced stages are observed in which the dolomite
crystals have penetrated the surrounding coatings.
3. Pellet grains are common and are frequently associated with skeletal
grains and algal masses. Pellets are oolite-like grains without internal structure
and vary in size from very fine to fine grain. They are of uniform size in any
given occurrence. These grains have been referred to as pseudo-oolites by some
workers.
4. Foraminiferal grains (almost exclusively Miliolidae) are common
throughout the Dollar Bay Formation. They usually are scattered among other
grains, but sometimes occur alone in a matrix of calcilutite. On rare occasions
this grain type becomes the major constituent of the rock. The Miliolidae vary in
size from fine to medium grain and are invariably white in color.

BUREAU OF GEOLOGY

5. Detrital grains (derived from pre-existing rocks) and lumps (aggregates
of grains) were not observed.
6. Algal masses were occasionally seen in core chunks; in cuttings this rock
type is almost impossible to identify with a 10-power microscope.

DOLOMITE

Dolomite is of two types. One type is evaporite-associated, and is invariably
microcrystalline or very fine crystalline; the crystal structure is usually euhedral
(appendix 1). This type may be brown, tan, cream, or light gray in color; when it
is occasionally argillaceous, the color is gray. Argillaceous dolomite sometimes
occurs as a matrix within bedded nodular anhydrite (described below). It usually
occurs immediately above or below the cycle-bounding anhydrife beds.
Presumably in some areas the evaporite-associated facies is the stratigraphic
equivalent of a down-dip anhydrite but this was not demonstrated in available
control. Though effective porosity is not characteristic of evaporite-associated
dolomite, when it occurs it is intercrystalline.
The second dolomite type is usually secondary in origin, as shown by
preserved limestone grain structures. The crystal structure of dolomites of this
type is usually euhedral. Crystal size varies from micro to fine crystalline. Much
of the finely microcrystalline euhedral dolomite in the Dollar Bay contains
chalky intercrystalline porosity which is non-effective for oil production. Coarse
microcrystalline euhedral dolomite, as well as very fine and fine crystalline
euhedral dolomite, often contain pinpoint porosity, intercrystalline porosity, or
very fine to coarse size vugs which frequently are fossil molds.
Anhedral dolomite (appendix 1) is not common. It is characteristically
non-porous, but on occasion may contain a small vug or two.

ANHYDRITE

Anhydrite occurs in five distinct forms; two as beds and three as
inclusions-in-carbonate.
Of the two bedded types, the most common has the appearance of an
amorphous translucent mass. It can be dark brown, dark gray, brown, tan, gray,
or white; on one occasion a blue color was observed. In the other bedded type,
nodules comprise about 95 percent of the rock. This type of anhydrite usually
has a matrix of brown microcrystalline dolomite between the nodules, and
apparently it is near the terminal stage of the replacement of carbonate by
anhydrite.
The three inclusion-in-carbonate types of anhydrite are vastly different from
one another. In the firs, or isolated nodular type, the nodules vary from 1/16 to
1% inches in diameter and are more often found in calcilutitic limestone than in

SPECIAL PUBLICATION NO. 15

calcarenitic limestone. This type is usually white and amorphous, but
occasionally may be clear and crystalline. It was observed to grade into the
bedded nodular type of anhydrite described above.
A second type of anhydrite inclusion occurs as either a mass of brown,
tabular, resinous, translucent, rectangular crystals, or as individual crystals.
These crystals average about 1/16 inch in diameter and inch in length, and are
imbedded in light-colored limestones, or microcrystalline dolomites. If sliced at
the proper angle, the individual crystal form closely resembles a dolomite
rhomb.
Lastly, a rare form of anhydrite inclusion occurs as clear 1/32-inch spheroids
scattered uniformly throughout either dolomite or calcilutitic limestone. These
spheroids, when dissolved (sometimes by drilling processes), leave a distinctive
non-connected pinpoint porosity.
CLASTICS

Shale is rare. It usually occurs as a contaminant, in the form of dark partings,
varves, or disseminated argillaceous material within limestone or dolomite.
Sandstone was not observed.

TYPICAL DEPOSITIONAL CYCLE IN THE LOWER CRETACEOUS SECTION

A study of the cores of the Big Cypress Group reveals the same cyclic
sequence of rock types repeated many times. A complete cycle was nowhere
observed, and occasionally only one bed of the cycle was present. Cycle thicknesses
varies from 10 to 400 feet, with the typical cycle being about 50 feet thick.
Porosity usually is found in the middle of the cycle, but on occasion it occurs
in other portions.
Figure 6 is a graphic presentation of the typical depositional cycle as pieced
together from many wells; below is a description of the cycle, with diagnostic
characteristics of the lithology underlined. Although the sequence is presented in
the order in which it would be drilled, the beds are numbered in the order of
deposition.

ENVIRONMENTAL IMPLICATIONS

The cyclic occurrence of carbonate and anhydrite rocks in the Dollar Bay
Formation indicates deposition in a relatively shallow water environment,
probably behind a barrier reef along the West Florida Escarpment. As sea level
fluctuated, different shallow water environments moved back and forth across
the broad Florida-Bahama Platform. A small rise or fall of sea level would cause
a given environment to move a long distance in a short interval of time. Five
major environments were identified in the Dollar Bay Formation. Their
characteristic lithologies are:

The shallow shelf environment produced a light-colored calcilutite containing
many miliolids and much chalky porosity. Microcrystalline secondary dolomite
with good porosity also is present. Anhydrite is rare to absent.
The deeper shelf environment resulted in the deposition of a light-colored
calcilutite containing skeletal, oolitic, pellet grains and Miliolidae; this rock
usually has chalky porosity. In addition, microcrystalline secondary dolomite
formed in this environment. The dolomite crystals are generally slightly coarser
than are those on the high shelf, and porosity is usually good. Dolomite also
occurs as patches in limestone, or as scattered dolomite crystals.
The shelf-basin transition environment produced a calcarenite containing
oolitic or skeletal grains. This rock has little calcilutitic void filler; therefore
porosity is usually good. Miliolidae are less common than on the shelf and
frequently occur as oolite centers. Dolomitization of calcarenites in this zone
results in a coarser structure than is characteristic of the shelf dolomites. This
coarse structure frequently results in good intercrystalline or vuggy porosity.
Colors are typically light to medium dark.
The shallow basin environment produced dark calcilutite with subordinate
oolitic or skeletal grains. This limestone occasionally contains brown anhydrite
crystals or white anhydrite nodules. Anhedral or euhedral microcrystalline
dolomite occurs occasionally and is usually non-porous.
The deeper basin environment resulted in the deposition of thick beds of
anhydrite, dark calcilutitic limestone and dark anhedral microcrystalline
dolomite. Lack of porosity is characteristic of carbonates in this environment.

PRODUCTIVE CHARACTER

POROSITY

Each unit of the Dollar Bay Formation contains porosity. This is mapped as
equivalent good porosity (appendix 1).
The five types of porosity which occur in the Dollar Bay Formation, arranged
in the order of quantitative occurrence, are:
1. Chalky porosity Unfortunately, chalky porosity is of no economic use as
an effective oil reservoir rock, since its extremely fine size causes it to be
impermeable to oil. It is usually intercrystalline in nature. On electric logs it is
characterized by a high spontaneous potential and a very low resistivity. This
porosity is characteristically so fine that it is frequently invisible using a
10-power microscope. Though it is always impermeable to oil, it is not
impermeable to water. In the Sunoco-Felda field, for example, water-bearing
chalky porosity forms the upgradient terminus for the productive Roberts zone
of the Sunniland Limestone. Chalky porosity is found in either calcilutite or in
finely microcrystalline dolomite and mainly in the shallow or deeper shelf
environments.

SPECIAL PUBLICATION NO. 15

As chalky porosity is non-effective, it is not included in the porosity thickness
figures of the equivalent good porosity maps in this report.
2. Intercrystalline porosity This type is found in euhedral dolomites. Crystal
sizes and pore spaces range from the upper end of the microcrystalline size to
the fine crystalline size. Intercrystalline porosity values can be high.
3. Pinpoint porosity. This is actually a type of vuggy porosity in which pores
are of a micro to very fine size. When found in sufficient quantity (10 percent or
more bulk volume), they are connected and can form an effective petroleum
reservoir. Pinpoint porosity is usually found in secondary dolomites or
calcarenitic limestone.
4. Vuggy porosity (including fossil molds). This type of porosity is not
common in the Dollar Bay Formation, although it occasionally occurs in some
dolomites. 'Vuggy porosity mainly has served to improve the permeability of
other porosity types.
5. Intergramilar porosity. This porosity occurs mostly in oolitic or miliolid
calcarenites. In calcarenites of pellet or skeletal grains, the primary intergranular
porosity was usually destroyed by secondary comentation.

SOURCE ROCK

Since there is little shale in the Dollar Bay Formation, the dark carbonates
must have served as source beds for petroleum.
The occurrence of many thin anhydrite beds throughout the section blocking
vertical migration, and the lack of laterally widespread beds of porosity,
indicates that a given cycle to be favorable for the production of oil, must have
source beds and porosity beds in close proximity.

TYPE WELL FOR THE DOLLAR BAY FORMATION

The type well for the Dollar Bay Formation is here designated as the Humble
Oil and Refining Company, No. 1 Collier Corporation well which is a dry hole
located in Sec. 27-T50S-R26E, Collier County (well 32). The interval designated
as the Dollar Bay Formation was cored with a diamond bit, and extends from
9,850 to 10,360 feet, core depths (9,851 to 10,358 feet, electric log depths).
The name, suggested by W: R. Oglesby of the Bureau of Geology, was taken
from Dollar Bay,located in Sec. 27-T50S-R25E just south of the town of Naples,
and some 6 miles from the .type well. Data on this well, including the cores, are
indexed as accession No. W-2420 in the files of the Bureau of Geology.

BUREAU OF GEOLOGY

TYPE WELL LITHOLOGY

The type well description is presented in tabular form in order to facilitate
reference. Although it is arranged in the order of penetration, discussion of the
stratigraphy is in the order of deposition.
Figure 7 is a previously unpublished generalized electric-lithologic log of the
Dollar Bay Formation in the type well as prepared by W. R. Oglesby.

REGIONAL STRATIGRAPHY OF THE DOLLAR BAY FORMATION

The combinations of facies maps presented in this report summarize the
lithologic information needed for reconnaissance petroleum exploration. As
some of the maps presented are not in common usage, explanations of the use
and basis of construction are presented in appendix 2.

LITHOLOGY

The Dollar Bay Formation consists of limestone, dolomite and anhydrite and
is subdivided into four units: A, B, C and D. Units A, C and D occur as single
sedimentary cycles. Unit B is multicycled. The gross lithology of this formation
differs from that of the other parts of the Big Cypress Group in being
represented mostly by the chalky carbonate of unit C.
Dark rocks are concentrated in the lows and light rocks on the highs; the color
map (Map 1) strongly suggests that anaerobic lagoonal conditions may-have
existed in the- area ofrHendryy ad dCoier counties. Dolomite, although
concentrated on the Martin-Largo axis, (Map present throughout the study
area. Limestone is concentrated in the lows. Anhydrite tends to concentrate in
the lows and thin over highs, but on the Largo high there is an anomalously large
concentration of anhydrite. Porosity (Map 3) is concentrated on the highs,
especially along the Martin-Largo-Pine Key trend.

THICKNESS

The Dollar Bay Formation is 505 feet thick in the type well. In other parts of
the study area it ranges from 380 feet to 620 feet in thickness. The average
thickness is about 450 feet.

CONTACTS

The top of the Dollar Bay Formation is placed at the top of a persistent
anhydrite bed about 30 feet thick; this is also the top of unit A. Conformably
overlying this marker are unnamed carbonates of probable Washita Age. The
Dollar Bay Formation rests conformably on an unnamed thick anhydrite of
probable Trinity Age.

Map 3. Thickness of potential porosity and equivalent good porosity in
the Dollar Bay Formation.

BUREAU OF GEOLOGY

PRODUCTIVE CHARACTER

The Dollar Bay Formation is second only to the Sunniland Limestone of
TrinityAge as a prospective horizon for oil. Appendix 3 lists shows observed by
the writer in his lithologic studies of 17 wells; the many additional shows
included on the maps were obtained from numerous sources.
Twenty-six shows are presented on the maps of the units of the Dollar Bay
Formation. Of these, one consisted of 15 feet of oil recovered on a 20-hour drill
stem test, two consisted of oil-cut mud recovery on drill stem test, two were
good oil shows in cores or cuttings, and 21 were poor shows reported in cores or
cuttings. Most of these 26 shows are in unit C.
The percentage of source-type dark limestone in the section varies from 10 to
75 percent; source-type rocks are especially well developed in unit C.
Equivalent good porosity in the Dollar Bay Formation ranges from 10 to 120
feet in thickness.
The favorable area map for the Dollar Bay Formation was made by combining
the favorable area maps of its four units. The greatest number of favorable areas
are found in an area around the northeastern end of Lake Okeechobee (Map 4).

UNIT D

LITHOLOGY

The distinguishing characteristic of this unit is the unusually large percentage
of porous calcarenite it contains; frequently, dark porous dolomite has partly or
entirely replaced the calcarenite.
Unit D occurs as a single sedimentary cycle in which carbonates predominate.
The limestone is generally calcarenitic with skeletal, oolitic and pellet grains;
occasionally Miliolidae and rudistid fragments are present. Calcarenitic material
also occurs in a matrix of calcilutite; a few black structureless grains are present
in the' calcilutite of the northern portion of the study area. In color the
limestone ranges from black through light gray and cream.
Dolomite is usually brown, gray or tan in color. In general it is
microcrystalline, though occasionally it is very fine or fine crystalline. Euhedral
crystal structure is the most common form, but the anhedral form frequently
occurs. Completely dolomitized porous calcarenite is common, as is dolomitic
limestone.
Anhydrite occurs as the bedded variety; in color it is dark gray, brown, tan
and white. Rarely white nodules or brown crystals of anhydrite occur in the
carbonates.
Argillaceous material is rare.

Map 4. Composite favorable area map of units in the Dollar Bay
Formation.

BUREAU OF GEOLOGY

The basal portion of unit D consists of a bed of dense calcilutite 30 feet in
thickness. As this lithologic unit is similar in appearance on electric logs to an
underlying anhydrite, care must be taken in picking the anhydrite top on electric
logs. Dark carbonates (Map 5) are concentrated in the lows, and light carbonates
are concentrated on the highs. Limestone (Map 6) is present everywhere, but is
concentrated in the lows. Anhydrite is widespread, thinning over highs and
thickening in lows. Porosity (Map 7) is widespread, and good porosity is
common in limestone.

THICKNESS

In the type well unit D is 56 feet thick. In other parts of the study area it
ranges from 40 feet to 80 feet. The average thickness is about 55 feet.

PRODUCTIVE CHARACTER

Although five oil shows were reported from unit D, all are poor in quality.
Effective intergranular and pinpoint porosity are widespread and are found in
calcarenite. Chalky non-effective porosity also occurs. Dolomite in unit D
frequently contains very good intercrystalline, pinpoint, or vuggy porosity.
Source beds are widespread. The area of favorable juxtaposition of source and
reservoir beds occurs northeast of Lake Okeechobee (Map 8).

UNIT C

LITHOLOGY

The distinguishing characteristic of this unit is its great thickness of chalky
carbonate, both light-colored calcilutite and microcrystalline dolomite.
This unit was deposited in a single sedimentary cycle. The limestone is a
calcilutite, usually chalky, and is accompanied by occasional streaks of
calcarenite. Limestone grains are skeletal, pellet or oolitic; Miliolidae are
common. Colors range from dark brown through cream. In the vicinity of
Hendry County thick beds of dark, dense calcilutite occupy most of the unit
(Map 9). This rock is frequently petroliferous. In one instance it yielded a small
quantity of oil, thus strongly indicating that this rock type is the source of the
numerous oil shows observed in unit C.
Dolomite colors range from dark brown through cream. Texture is
microcrystalline, very fine crystalline, or fine crystalline. Crystal structure is
usually euhedral, but may occasionally be anhedral. Completely dolomitized
calcarenite is common.

Map 7. Thicknesses of potential reservoir and equivalent good porosity
in unit D.

BUREAU OF GEOLOGY

SOi I recovery on DST
QOil cut mud on DST
3 Good show in rock
OPoor show in rock
0 No show reported

@ Lithologic Control Wells
o040
0 41
0 2

10 0 t0 20 30 MILES
APPROX SCALE

Map 8. Favorable area map of unit D.

o*

So
C9 0 39a

SPECIAL PUBLICATION NO. 15

@Lithologic Control Wells

o400.o "iii1 KDark li in Carbonate

000-251
Contour Interval 20 Feet OJ: 25-50%
S50-75 %

10 0 10 20 30MILES E S
APPROX. SCALE

Map 9. Isopachous and percent dark carbonate map of unit C.

BUREAU OF GEOLOGY

Bedded anhydrite is the dominant type and is dark brown, brown, dark gray,
grayish tan and white. White nodules and brown crystals of anhydrite occur
occasionally in the carbonates.
Argillaceous material is rare in this unit.
Light rocks are present throughout the study area, although they are more
prevalent over the highs. Dark rocks occur in all parts of the area, but are most
commonly found in the lows. Very large thicknesses of dark carbonates are
present in the area of Collier and Hendry counties. Perhaps a local anaerobic
lagoon developed in the Collier-Hendry area during the long stable period in
which unit C cycle was deposited. Anhydrite (Map 10) is widely distributed,
tending to thin over highs and thicken in lows. Dolomite (Map 10) is more
common over highs, and limestone predominates in the lows. Porosity (Map 11)
is widely distributed, but it is best developed over the highs.

THICKNESS

In the type well unit C is 341 feet thick. In other parts of the study area, it
ranges from 260 feet to 420 feet. The average thickness is about 325 feet.
Within the upper Lower Cretaceous section down through the Punta Groda
Formation (Frontispiece), there are only two cycles of sedimentation more than
200 feet thick. The oil productive Sunniland Limestone is one of these and unit
C is the other. From this limited evidence it would appear that thick cycles are
much more favorable for oil accumulation than are thin cycles.

PRODUCTIVE CHARACTER

Sixteen shows of oil have been reported in this unit: one consisted of 15 feet
of oil recovered on a 20-hour drill stem test, two consisted of oil-cut mud
recoveries on drill stem test, two were good shows in cores and cuttings, and 11
were poor shows in cores and cuttings. The 15 feet of oil was recovered from
dense, dark, source-type rock with little effective porosity. Unit C is the only
unit in the entire Big Cypress Group to yield free oil on drill stem test.
Much chalky, non-effective porosity commonly occurs in the calcilutites and
microcrystalline dolomites of unit C. The occasional calcarenites usually have
good intercrystalline, pinpoint or vuggy porosity.
In the Humble, No. 1 State 1004 (well 25), a core of unit C contained
numerous beds of oil-saturated, microcrystalline, chalky dolomite without
effective porosity separated by beds with effective porosity. These porous beds
did not contain oil shows. This condition suggests that oil has migrated through
the area, and may be trapped in an updip reservoir.
This unit has the most extensive favorable area in the Dollar Bay Formation.
The best area, that in which source rock and porosity are coincident, occurs

Map 11. Thicknesses of potential reservoir and equivalent good
porosity of unit C.

SPECIAL PUBLICATION NO. 15

around Lake Okeechobee (Map 12); this area is near the main concentration of
source rock in Hendry County. Another less attractive favorable area lies to the
south on the flanks of the Largo high.

UNIT B

LITHOLOGY

The distinguishing characteristic of this unit is the presence of many
incomplete cycles, perhaps as many as six.
Carbonate is the main constituent of the unit, but anhydrite is common.
The limestone is generally calcilutitic, and occasionally contains skeletal,
pellet or oolitic grains. Miliolidae are common. Colors range from dark brown to
light gray and cream (Map 13).
Dolomite colors include brown, tan, dark gray, gray, light gray and cream.
Textures are microcrystalline and very fine crystalline. Both euhedral and
anhedral crystal forms occur. Completely dolomitized calcarenites are common.
Bedded anhydrite, the common variety present, is brown, dark gray, and
white. White nodules and brown crystals of anhydrite occur occasionally in the
carbonates.
Argillaceous material is common as a contaminant in the carbonate rock. The
color map shows a concentration of dark beds in the area of Hendry and Collier
counties similar to the concentration of dark carbonates in unit C. In other parts
of the study area, dark rocks of unit B mainly occur in lows, and light rocks are
concentrated on highs. Limestone (Map 14) is concentrated in the lows, and
dolomite on the highs. Anhydrite is restricted to the area in and bordering the
Broward trough; it thins over the Forty Mile Bend high. Porosity (Map 15) is
confined to the high areas, and is best developed on the Martin high.

THICKNESS

In the type well, unit B is 53 feet thick. In other parts of the study area, it
ranges from 40 to 75 feet. The average thickness is about 55 feet.

PRODUCTIVE CHARACTER

Three shows of oil have been reported from unit B, one good and two poor.
Effective porosity is restricted entirely to dolomite, and is pinpoint or
intercrystalline in nature. Chalky non-effective porosity is found occasionally in
calcilutites and microcrystalline dolomites. Possible source beds in the form of
dark carbonates are distributed fairly widely (Map 16).

BUREAU OF GEOLOGY

MAN ATEEI HARD E
SE" ST. LUCIE

SARASOTA DE 4

Favorable Area

040

042

Map 12. Favorable area map for unit C.

P 2 3.0 MILES
APPROX SCALE

SPECIAL PUBLICATION NO. 15

EXPLANATION
Oil recovery on DST
SOil cut mud on DST
o Good show in rock
D Poor show in rock
0 No show reported
SLithologic Control Wells
040
0421
042

Map 15. Thicknesses of potential reservoir and equivalent good
porosity in unit B.

BUREAU OF GEOLOGY

Favorable

* 99A

040
041
042

10 0 o0 20 30MILES
APPROX.SCALE

Map 16. Favorable area map of unit B.

SPECIAL PUBLICATION NO. 15

Unit B can be considered only a fair horizon for oil occurrence, since the areas
of favorable juxtaposition of porosity and source-type beds is small. The small
size and sinuous shape of these areas suggests that their location is doubtful since
a minor shift in the bounding contours could eliminate them.

UNIT A

LITHOLOGY

The distinguishing characteristic of this unit is its widespread content of
porous dolomite or calcarenite.
Unit A consists of one sedimentary cycle composed of carbonate and
anhydrite in approximately equal amounts. Occasionally Miliolidae are an
important constituent. Calcilutite is subordinate, frequently containing scattered
grains of the types described. The colors are black, brown, tan and cream.
Dolomite has a wider variety of colors, ranging from dark brown and dark
gray to light gray and cream. Texture is microcrystalline or very fine crystalline;
the crystal form is usually euhedral, but occasionally may be anhedral.
Completely dolomitized calcarenites are common.
Bedded anhydrite is the most common type and is brown, gray, tan and
white. White nodules and brown crystals of anhydrite are sometimes present in
the carbonates (Map 17).
Argillaceous material is rare.
Light rocks tend to be concentrated in the highs and dark rocks in the lows.
The concentration of dark carbonates in the Hendry County area indicates that
this area may have been an anaerobic lagoon, similar to that postulated during
the deposition of units B and C. Dolomite (Map 18) is concentrated on the
highs, and limestone in the lows. Anhydrite is widely distributed, thinning over
highs and thickening in lows. In unit A the area of rocks with good porosity
(Map 19) is extensive.

THICKNESS

In the type well unit A is 60 feet thick. In other parts of the study area, it
ranges from 30 to 60 feet. The average thickness is about 45 feet.

PRODUCTIVE CHARACTER

Only two poor shows of oil have been reported from unit A. Good effective
porosity occurs in dolomite, and also in calcarenitic limestone. Porosity is
intercrystalline and pinpoint in dolomites, and intergranular and pinpoint in
calcarenites. Chalky, non-effective porosity occurs occasionally in calcilutite and
microcrystalline dolomite. Source beds are widespread and of fair quality.

BUREAU OF GEOLOGY

.*.*

EXPLANATION
SilE recoveryon ST.ST

0ilcutmudonDST .:::.:.- ..
0 Good show in rock
Poor show in rock
o No show reported
SLithologic Control Wells :::: ::
: I:::::: ::::::.7 :

eOil recovery on DST
0Oil cut mudon DST
(Good show in rock
OPoor show in rock
0 No show reported
@Lithologic Control Wells

Equivalent Good Porosity
Contour Interval 25 Feet

0t 0 10 20 30MILES
APPROX.SCALE

Potential Reservoir Thickness
S0-25 Feet
25-50 Feet
E > 50 Feet

Map 19. Thicknesses of potential reservoir and equivalent good
porosity in unit A.

SPECIAL PUBLICATION NO. 15

The occurrence of good porosity and fair quality source beds indicates that
unit A is a good prospective horizon for the occurrence of oil.
The favorable area for unit A (Map 20) is long and sinuous. This shape
indicates some unreliability of boundaries, since a minor shift in contours might
eliminate the favorable area.

DISCUSSION OF CROSS-SECTIONS

The four stratigraphic cross-sections of the report as shown in the index map,
figure 8, were designed to show lateral changes in lithology and thickness across
the more prominent structural features of the study area.
Section A-A', figure 9, is an east-west section crossing the north flank of the
South Florida Basin. The upper and lower contacts of the Dollar Bay Formation
are on top of persistent anhydrite beds which exhibit high resistivity kicks on
the electric log. Dolomite percentage increases in an easterly direction onto the
Martin high. The porous beds do not appear to have regional continuity; this
could result in the development of stratigraphic traps. The thick interval in wells
1 and 6 suggests that subsidence was taking place in the area of these wells.
Section B-B', figure 10, is also an east-west section extending from the South
Florida Basin center eastward to well 25, which contained numerous good oil
shows. Well 27, located in the area of the postulated Hendry County lagoon,
contains source-type limestone between 9700 and 9900 feet. To the west, this
rock type becomes a chalky white limestone, while to the east it becomes a light
colored, chalky limestone and dolomite.
Section C-C', figure 11, is a north-south section extending from the Martin
high (well 6) to the Marquesas Keys (well 41). From the upper end of the Martin
high (well 6) the Dollar Bay Formation thickens in a southerly direction to well
7, also on this feature. Southward wells 25 and 33 show no change in thickness
from the Martin high to the Forty Mile Bend high (well 33). Southwest from this
feature, the formation thickens steadily to the vicinity of the northern flank of
the Pine Key arch (wells 39 and 41). Here the section attains the greatest
thickness so far observed in this formation. Although dolomite is common in all
the wells of this cross-section, there is more of it on the Martin high at the north
end (wells 6 and 7) than anywhere else. Porosity, in discontinuous beds,
decreases sharply away from the Martin high; wells 33 and 39 contain little
effective porosity.
Section D-D', figure 12, is a northwest-southeast section which starts on the
shelf (well 3), crosses the lagoonal source-facies area (wells 10, 20 and 27),
continues across the Forty Mile Bend high (well 33), and terminates on the
Largo high (well 37). In well 37, the Dollar Bay interval is noticeably thinner
than in other wells in this area. Porosity, in discontinuous beds, is greatest on the
shelf to the northwest (well 3), and on the Largo high to the southeast (well 37).

BUREAU OF GEOLOGY

M A NATEE H A RD E E

---------- HIGHLANDS
S L._._j I .

Favorable Area

C9

o 40

04241

10 0 10 20 30MILES
APPROXSCALE

Map 20. Favorable area map of unit A.

: o 9~
; o CO
0.^ c=:

SPECIAL PUBLICATION NO. 15

10 0 10 20 30MILES
APPROX.SCALE

Figure 8. Index map showing location of cross-sections.

C 042

BUREAU OF GEOLOGY

< 0N .2-
<0 c

< )
Sc

< o
Z )

-J
C-
z
w
OK

z
0

w
-J

I

0
-10 04)
o
aU) C
"Mm

(2
z

0

4-

U_

o
0
c

0 O E
0

U 0

.rE
I-

aOO

0 .-

E 5
0 CL
H C .
S.J W

HIEL

SPECIAL PUBLICATION NO. 15 55

W-1 000

So- w 11 reooea

S-- oF I I

zo
o Oo
0 3

_o

a -- <

IZ o -
z z

'- |

Z 000 '1 00O1 000,1
3 Of . ..

z (-)o

BUREAU OF GEOLOGY

d
(J

o
L)
U
43)
0
C
0
2

d
U
2)
(U

0

u

m
Eo
'o

m

d
U

u
rU
2)

m

E
(U
0-

00E6 0I0g0 I 000,11

T

VU \I

100g6UU u00 0

=II 501,l

1'CD

-J
'L

I r- ~

D 6 00001 ..'. I

O4

S098 0006 0096

'~vlLj^ ^^- J ^

U) d

SCM-
U< 0
,o cr U N U
2 6 M -'

I-

O E

oo'f
0 0
0. O* 0
* e@

0)

- aE
0 .- C. .
E "' o
o. .-
Cr _, Iq

oU,

(. 0 .
.Z

Cl)
-J r-
LLj

0 o0r
UZ
Z)
-J

Ll-
00 _J
8

I
"I-
< o
wuo
Sno 0
S1 -

0

0 -I 0

3:Z 2)
-J

oo,
2
<

V VW11 'V l

I~ '

~"~

I

L-- -1----LH

I

SPECIAL PUBLICATION NO. 15 57

Ul

Eo
C)

S- 0 0009 00 OO 0006

z s- -

o C,
-) .

o 0C
z E 0 0096 000 0 o o so.
E /

w = co
I- -

Z WI 0006 1 00os6 flflF 1Ei" 0
0 30 0
t o Z m 2 E

0 i . c m. . ..HHHHH.Hf l 0
o |

I) E 'a

Z ba 000 0096 01

0c z L

C- / 1 0

BUREAU OF GEOLOGY

In the intermediate wells porosity is scarce. The carbonate on the Largo high
(well 37) is composed almost totally of dolomite; in the other wells limestone
predominates. In well 37, anhydrite is much thicker in the Dollar Bay Formation
than in any other well on the four cross-sections. The oil shows in the wells of
cross-section D-D' are all in the tight dark carbonates considered to be source
beds, except for the top-most show in well 10 which occurs in porous dolomite.
While all of the wells in the center of the cross-section are of nearly uniform
thickness (wells 10, 20 and 33), well 27 is anomolously thin in the Dollar
Bay Formation.

SUMMARY OF THE OIL POTENTIAL OF THE
DOLLAR BAY FORMATION

The Dollar Bay Formation has the greatest potential for oil production within
the Big Cypress Group. This is due to the presence of unit C.
It is believed that stratigraphy, as much as local structure, will control
commercial oil occurrence in the Dollar Bay Formation. Stratigraphic change in
association with local nosing accounts for the trapping of Sunniland Limestone
oil in the Sunoco-Felda field of Hendry and Collier counties (fig. 2).
Since closed structure has proven difficult to find in south Florida, and in fact
may be rare, regional nosing will be important in localizing oil accumulations.
Small displacement faults may also contribute to the entrapment of oil.
A barrier reef may be coincident with much of the Florida escarpment, 120
miles to the west. The escarpment extends from DeSoto Canyon at the north,
southward into the Florida Straits. Recent seismic profiles and a dredged sample
(Antoine et al, 1967, fig. 3, p. 259) suggest that a local anticlinal structure
superimposed on the escarpment is of Cretaceous age, and may be a reef. The
rocks of the onshore portion of the South Florida Basin appear to be a back reef
facies of this possible barrier reef. Since there was no large actively sinking
hingeline in the onshore portion of this basin, it is unlikely that vertical reef
development has occurred here. If onshore reefs exist, they should be low in
relief, and probably will be as much biostromal as biohermal. Thus they are not
expected to form a trapping structure, but should contribute greatly to the
quality of the reservoir.
Offshore, greater possibilities exist for the development of a major reef trap in
the barrier reef of the Florida escarpment. An atoll may have developed around
two possible basement highs indicated on the marine gravity map of Dehlinger
and Jones (1965, P. 104).
A graben is interpreted about midway between the southwest Florida coast
and the escarpment (Antoine and Ewing 1963, p. 1977) on the basis of seismic
refraction work. To produce such a feature, major structural movement would
be required.

SPECIAL PUBLICATION NO. 15 59

Most of the oil tests in south Florida have been drilled to test the Sunniland
Limestone. The area favorable for Sunniland production nowhere coincides with
the area favorable for Dollar Bay production; therefore very few tests have been
drilled in the favorable area of the Dollar Bay Formation.
The bands of favorable area in the Dollar Bay Formation are located updip
from a similar band of favorable area in the older Sunniland Limestone. This
updip migration indicates a broadening of the basin during the continuous
transgression of the Lower Cretaceous sea.
Geophysical exploration in the Dollar Bay favorable area may indicate the
presence of a structural feature which would help localize a stratigraphically
controlled trap. Appendix 8 gives a brief summary of past exploratory activity
and geophysical problems in south Florida exploration.

1. Calcarenite. A limestone with over 50 percent sand-sized grains.
2. Calcilutite. Micro-structured limestone with less than 50 percent sand-sized grains;
usually a consolidated lime mud. Occasionally this rock may be a completely
recrystallized calcarenitic limestone in which all original texture was destroyed, only
"ghosts" or color differences remaining to indicate original fossil fragments.
3. EuhedraL Refers to dolomite crystal structure in which individual crystals are separate
and can be seen quite plainly. Sucrosic dolomites are always euhedral, but euhedral
dolomites are not always sucrosic.
4. AnhedraL Refers to dolomite crystal structure in which individual crystals are
interlocked; this obscures the crystal boundaries almost to oblivion.
5. Equivalent good porosity. Since thick sections of poor porosity do not have the oil
productive quality of thinner sections of good porosity, a map of un-weighted porosity
thickness is completely misleading. To bring porosities of similar thicknesses but
varying quality into some sort of equivalency for mapping purposes, the following
formula was used:
Footage of poor porosity (0-5%) divide by 4
Footage of fair porosity (6-10%) divide by 2
Footage of good porosity (11-15%) divide by 1
Footage of excellent porosity (15-30%) divide by 1
Equivalent good porosity is the sum of the respective weighted equivalencies as derived
above. This formula provides porosity figures for exploratory mapping which are
realistically comparable.
6. Potential porosity. Although rock capable of developing porosity may not be porous at
the immediate locality of the well, it is still important to know the thickness of rock
which is capable of developing porosity. A cemented oolite is an example of a rock
type having potential porosity. As mapped, potential porosity also includes rock which
is actually porous.
Thus the potential porosity maps in this report show by pattern the thickness of rock
that has a potential of developing porosity. This map is overlaid with the equivalent
good porosity map, which shows by contours the weighted thickness of porosity at
points of controL

SPECIAL PUBLICATION NO. 15

APPENDIX 2

SPECIAL PUBLICATION NO. 15

EXPLANATION OF MAP CONSTRUCTION-

The faces maps of this report have been combined in combinations to
facilitate their use, and to reduce the number of illustrations. The isopachous
and percent dark carbonate maps were combined, as both tend to show general
basin form such as shelf and basin center. The percent dolomite in carbonate and
the percent anhydrite maps were combined since they both deal with the
lithologic composition of the mapped unit. The potential porosity and
equivalent good porosity maps were combined because they both deal with
reservoir conditions. The favorable area map stands alone.

ISOPACHOUS MAP

Contours are used to show thickening, thinning and extent of the mapped
unit. In local areas this may indicate the presence of features such as anticlines
or reefs.

PERCENT DARK CARBONATE MAP

This map, known as a color map, shows the percentages of dark carbonate by
a series of patterns. In rock sections exclusively of carbonate, the source for oil
must be within the carbonate rock itself. The colors most closely associated with
organically rich beds are black, dark gray or dark brown; therefore, a map
showing the percentage of these colors can be interpreted as showing the
potential source capability of the unit.

PERCENT DOLOMITE IN CARBONATE MAP

This is merely a dolomite-limestone ratio map expressed as percentage for ease
in understanding. It shows by patterns the progressive change from dolomite to
limestone. As porosity in the Dollar Bay Formation most frequently occurs in
dolomite, it is useful to know the location of high concentrations of dolomite.

PERCENT ANHYDRITE MAP

Contours are used to show the percentage of anhydrite in the mapped unit.
Anhydrite is the third major lithologic constituent in the Dollar Bay Formation.

POTENTIAL POROSITY MAP

A progression of patterns is used to show the thickness of rock that
potentially could develop porosity (see appendix 1).

BUREAU OF GEOLOGY

EQUIVALENT GOOD POROSITY MAP

The thickness of weighted equivalences of good porosity, derived from the
formula explained in appendix 1, is shown by contour on this map.

FAVORABLE AREA MAP

A favorable area is usually sought by first studying the rocks where oil is
produced, and then looking for areas where similar facies exist. In the case of the
Dollar Bay Formation, this method cannot be used since there is no oil
production from which to start.
The favorable area maps are constructed by outlining the coincidence of
favorable porosity thickness, as shown on the equivalent good porosity maps,
and a reasonable amount of source rock, shown on the map of percent dark
carbonate.
Considering the depths at which beds of the Dollar Bay Formation occur, ten
feet of oil column was considered the minimum thickness desirable for
commercial oil production. For this reason, a minimum of ten feet of equivalent
good porosity was used as the lower porosity limit in constructing the favorable
area maps. In addition, it was decided by the writer that 25 to 50 percent dark
carbonate was adequate to serve as effective source rock. A higher percentage of
dark carbonates usually is associated with conditions which are not conducive to
the formation of porosity. On the other hand, the percentage used must be
sufficient to compensate for the fact that not all dark carbonates are source
beds.

The Poza Rica field, and the Golden Lane trend of fields are prolific
Fredericksburg Age producing areas of the Tampico embayment in Mexico.

POZA RICA

This field is a stratigraphic trap situated on the northeast flank of a southeast
plunging nose. The dip on the northeast flank of this structure is steep while on
the southwest flank it is gentle. The productive area covers some 20 square
miles. Poza Rica was discovered at a depth of 6700 feet in 1930 by drilling on a.
large gravity feature.
The productive horizon is the Tamabra Limestone, a transitional facies
between the non-porous Tamaulipas Formation to the west and the porous El
Abra Formation to the east. At Poza Rica, the Tamabra Limestone is some 650
feet thick. It is composed mainly of calcarenite containing skeletal grains of sand
and silt size. Compact non-porous zones of this type of limestone also occur
sporadically within the section. In addition, occasional beds of non-porous
calcilutite are present. Secondary dolomitization is widespread within the
section. Rudistid debris forms numerous shoals which are present horizontally as
well as vertically in varying number; the greatest cumulative thickness of these
occurs along the axis of the productive area, which is not the axis of the present
structure. Both updip and downdip, the highly porous rudistid shoals disappear.
Porosity varies from 8 to 20 percent and averages 14 percent. Permeability varies
from 5 millidarcies to 1000 millidarcies.
Current production from the Poza Rica field is 70,000 barrels of oil per day.
The cumulative production is 960 million barrels of oil. The calculated reserves
of the field equal 1,100 million barrels of oil.

GOLDEN LANE

The Golden Lane trend of fields is now known to form a giant atoll some 75
miles long by 35 miles wide. Production is from a series of biohermal crests on
this atoll. The productive trend is delineated by the vertical accumulation of
many rudistid shoals and bioherms in the El Abra Formation, which is as much
as 8000 feet thick. In the lagoonal area the El Abra is composed mainly of
miliolid limestone with some anhydrite; this description could also fit the Dollar
Bay Formation in south Florida. Detrital and bioclastic debrisfringe the atoll.Off
the west flank of the atoll dips are 30 degrees, while into the lagoon dips average
2 degrees. Depths to the productive horizon range from 1500 to 2500 feet.

80 BUREAU OF GEOLOGY

Current production from the Golden Lane trend varies from 35 to 40,000
barrels of oil per day. As of June, 1969, the cumulative production was 1,393
million barrels of oil; the remaining reserves of the field are estimated to be 239
million barrels of oil.